Download demo project (Display applet extension) - 14 Kb

Introduction

After writing my tutorial on property sheet shell extensions, I had some folks ask me how to go about customizing pages in Control Panel applets. The procedure is very similar to writing a "normal" property sheet extension, so I have written this article that summarizes how to do it.

I classified this article as intermediate level, as you should already be familiar with property sheet extensions, the COM interfaces involved and their methods, and how to code property pages in straight SDK calls. If you need a refresher, see my tutorial.

Also keep in mind that customizing Control Panel applets shouldn't be done lightly. If you're writing custom driver software (e.g., something like Microsoft's IntelliPoint or IntelliType), then it's perfectly reasonable. But if you just have a little wallpaper-switching program, you'd be better served to write your own configuration app instead of adding to the already-crowded Display Control Panel.

The sample project for this article is a simple extension that adds two custom pages to the Display applet.

The Extension Interfaces

As with a normal property sheet extension, a Control Panel extension implements two interfaces: IShellExtInit and IShellPropSheetExt. The sequence of method calls is a bit different from normal property sheet extensions. The sequence goes like this:

IShellExtInit::Initialize() is called first, but since there are no files to be selected, the pidlFolder and lpdobj parameters are both NULL. You can perform any one-time initialization in Initialize().
IShellPropSheetExt::ReplacePage() is called once for each page that is replaceable, if the applet allows pages to be replaced. You can replace one page in each ReplacePage() call; I'll explain this below.
IShellPropSheetExt::AddPages() is called once. You can add as many pages as you like in AddPages().

The big difference is the ReplacePage() method, which went unused in normal property sheet extensions. Certain Control Panel applets let extensions replace pages in the property sheet. You can find the exact pages and applets in the cplext.h file, but I've summarized them here:

Display applet: You can replace the Background page.
Keyboard applet: You can replace the Speed page.
Mouse applet: You can replace the Buttons and Motion pages.

The initialization interface

As I mentioned above, IShellExtInit::Initialize() is called, but since there is no selection in a Control Panel applet, the parameters are meaningless. If you have any one-time initialization to do, Initialize() is a good place to do it.

The property sheet interface

The Display applet allows extensions to replace one page (Background), so the IShellPropSheetExt::ReplacePage() method is called once. An extension can just return S_OK if it doesn't want to replace the page. If the extension does want to replace the page, it creates a new property page and the applet displays it in place of the built-in Background page.

The prototype for ReplacePage() is:

HRESULT IShellPropSheetExt::ReplacePage (
    UINT uPageID,
    LPFNADDPROPSHEETPAGE lpfnReplaceWith,
    LPARAM lParam );

The parameters are:

uPageID: A constant from cplext.h that indicates which page the applet is querying for. For example, the Display applet calls ReplacePage() with uPageID set to CPLPAGE_DISPLAY_BACKGROUND to indicate the extension can replace the Background page. In applets that allow more than one page to be replaced (such as the Mouse applet), ReplacePage() is called once for each page.
lpfnReplaceWith, lParam: lpfnReplaceWith is a function pointer that the extension calls to actually replace the page. lParam is a value that's meaningful to the shell and gets passed to the lpfnReplaceWith function.

The process for replacing a page is pretty much the same as adding a page. Our extension fills in a PROPSHEETPAGE struct, creates a page, and calls the lpfnReplaceWith function to replace the Background page. The sample page has no controls on it; the purpose is just to show how to get pages in the Display applet. I'm assuming that you're able to code a dialog proc for the page; if you need help with this, check out other articles on property sheets at CodeProject.

Here's the code for our ReplacePage() method. We first verify that uPageID is a value we expect.

#include <cplext.h>

STDMETHODIMP CDisplayCplExt::ReplacePage ( UINT uPageID, 
                                           LPFNADDPROPSHEETPAGE lpfnReplaceWith,
                                           LPARAM lParam )
{
    // Check that we're being called with the page ID we expect.
    if ( CPLPAGE_DISPLAY_BACKGROUND != uPageID )
        return S_OK;

We then fill in a PROPSHEETPAGE struct. The code here references the dialog proc and a callback function, which I'll get to in a bit.

PROPSHEETPAGE  psp;
HPROPSHEETPAGE hPage;

    // Set up the PROPSHEETPAGE struct.
    ZeroMemory ( &psp, sizeof(PROPSHEETPAGE) );

    psp.dwSize      = sizeof(PROPSHEETPAGE);
    psp.dwFlags     = PSP_USEREFPARENT | PSP_DEFAULT | PSP_USECALLBACK;
    psp.hInstance   = _Module.GetResourceInstance();
    psp.pszTemplate = MAKEINTRESOURCE(IDD_REPLACEPAGE);
    psp.pfnDlgProc  = ReplacementPageDlgProc;
    psp.pfnCallback = ReplacementPageCallbackProc;
    psp.pcRefParent = (UINT*) &_Module.m_nLockCnt;

We then create a page and pass it to the lpfnReplaceWith function.

    // Create the page & get a handle.
    hPage = CreatePropertySheetPage ( &psp );

    if ( NULL != hPage )
        {
        // Call the shell's callback function, so it adds the page to
        // the property sheet.
        if ( !lpfnReplaceWith ( hPage, lParam ))
            {
            DestroyPropertySheetPage ( hPage );
            }
        }

    return S_OK;
}

There are two additional functions, ReplacementPageDlgProc and ReplacementPageCallbackProc. You can check out the code in the sample project; they are just skeletons since the page has no controls.

Note that shell version 4.71 and later includes a Display extension that has a Background page replacement, so if you use the above code without disabling the shell's own extension, you'll still see a Background tab, as shown here:

[Our page with the shell's page - 13K]

An extension can also add any number of pages to the property sheet from its AddPages() method. This is done just as in normal property sheet extensions, and the code is almost identical to the ReplacePage() code above. Check out the sample project if you're dying to see the code.

The sample extension adds one page to the Display applet in AddPages(), and here are the final results:

[Our replacement page - 8K]

[Our additional display page - 7K]

Registering the Extension

The Control Panel applets that can be customized have their own area of the registry in HKEY_LOCAL_MACHINE. Unfortunately, the registry key used to extend the main Display property sheet is different on 9x and 2000, so we can't register our extension with an RGS script. On Windows 9x, it's HKLM\Software\Microsoft\Windows\CurrentVersion\Controls Folder\Display\shellex\PropertySheetHandlers, and on Windows 2000 it's HKLM\Software\Microsoft\Windows\CurrentVersion\Controls Folder\Desk\shellex\PropertySheetHandlers. We create a new key under PropertySheetHandlers for our extension in DllRegisterServer(), and remove the key in DllUnregisterServer().

I don't have an NT 4 system handy to check which registry key it uses, so for the time being the registration code treats NT 4 the same as Win 2000. I'd appreciate it if a kind reader could check on NT 4 for me, and let me know if the code is using the wrong key.

Debugging Control Panel Extensions

It's easy to debug a Control Panel extension in the Visual C debugger, once you know what debug settings to use. Microsoft Knowledge Base articles Q166168 and Q135068 have a good description of how to do it, but I'll give a quick summary here.

In your project settings, go to the Debug tab and set the executable to "rundll32.exe". The program arguments should be "shell32.dll,Control_RunDLL" followed by the full path to the CPL file that contains the applet. (Note that the "Control_RunDLL" part is case-sensitive.) It's not always obvious which CPL file to use, so here are some common ones:

Applet	Arguments (change "C:\windows\system" if you have Windows installed in some other directory)
Display	shell32.dll,Control_RunDLL C:\windows\system\desk.cpl
Mouse	shell32.dll,Control_RunDLL C:\windows\system\main.cpl
Keyboard	shell32.dll,Control_RunDLL C:\windows\system\main.cpl,@1

The "@1" part indicates which applet in main.cpl to run. (CPL files can contain more than one applet; adding @n runs applet number n, where n is a 0-based count.)

Here's a screen shot that demonstrates the settings, in case things still are a bit unclear:

[Debug settings - 7K]

When you start debugging, rundll32 will call the Control_RunDLL function in the shell, which in turn will launch the Control Panel applet. To end debugging, just close the applet's window.

Adding Custom Pages to Control Panel Applets